The process according to the invention thus relates to a process for enhanced separation of gold and silver from complex concentrated sulfide ores and concentrates. In addition to the primary metals, iron, cobalt, nickel and copper (zinc, lead), these complex ores contain the following constituents: arsenic, antimony, bismuth, selenium and tellurium.
Arsenic, antimony, bismuth, selenium and tellurium, both as such and together with sulfur, combined with the primary and/or noble metals, have a very adverse effect on the separation of gold and silver during the alkalic cyanide leaching of the ore or concentrate in the presence of oxygen. This adverse effect is due both to the solubility, in alkalic cyanide solutions, of the minerals which contain them, and also to their ability to form, on the surface of gold (silver), covering layers which prevent or inhibit cyanidation. When conventional processes are used, the concentrate or ore is roasted in order to eliminate the detrimental constituents and their compounds. However, often the roasting does not eliminate the covering-layer problems, and furthermore, it produces dense oxides which keep the noble metals enclosed, and soluble compounds which consume cyanides. The fly dusts which contain arsenic, antimony and bismuth are difficult to separate from the gas phase, highly toxic, and hazardous to the environment.
In the main, the processes for the treatment of low-grade gold and silver ores have remained unchanged for several decades.
The largest and richest low-grade gold ore deposits are found in South Africa. This discussion is primarily based on the data obtained from the refining of these ores [R. J. Adamson; Gold Metallurgy in South Africa, Johannesburg, 1972; P. J. D. Lloyd; Min. Sci. Engng. 10, 1978, 208-221]. The general features of Australian gold metallurgy are also discussed [K. J. Henley; Min. Sci. Engng. 7, 1975, 289-312, P. E. Clarke, N. Jackson, J. T. Woodcock; Australasian Inst. Min. Met. Proc. 191, 1959, 49-92]. Two principal categories can be distinguished in the South African gold ores, i.e. the Witwatersrand and the Barberton Mountain Land systems. In the former system, gold is present in quartz-serisite conglomerates and to a very small extent in sulfides or sulfates. In the latter system, gold and silver are present to a small extent in quartzes but in large amounts in conjunction with about 30 native metals or arsenides, antimonides, sulfides or sulfo-salts of metals (Cu, Fe, Ni, Co, Zu, Pb).
The treatment of gold and silver ores is primarily based on the following properties of these metals:
The high density of the native metals (Au, Electrum) and their compounds (density/compound: 16-19.3/Au(Ag), 15.5/Au.sub.2 Bi, 9.9/AuSb.sub.2, 9.1/Ag.sub.3 AuTe).
The low surface tension between gold and mercury (Hg wets gold and thereby binds it physically).
The solubility of gold, silver, their selenides and tellurides, and sulfides, in alkalic cyanide solutions under oxidizing conditions.
The conventional processing of gold/silver ores includes the following stages:
1. Ore crushing and grinding PA0 2. Concentration based on the specific gravity of noble metals PA0 3. Amalgamation of the concentrate obtained from stage 2 PA0 4. Froth-flotation of the residue obtained from stage 2 PA0 5. Roasting and washing of the froth-flotation concentrate from stage 4 PA0 6. Cyanidation of the calcine PA0 7. Filtration of the cyanide solution and precipitation of the noble metals PA0 8. Smelting of the noble metal precipitate (7) and of the distillation residue of the amalgam (3). PA0 (Fe,Co,Ni)(S,Se).sub.2 PA0 (Au,Pt)(As,Sb).sub.2 PA0 (Fe,Co,Ni)(As,Sb)S PA0 (Fe,Co,Ni)As.sub.2 PA0 Copper pyrite series, Cu(Fe,Ga,In)S.sub.2 PA0 Tin pyrite series, Cu.sub.3 (As,Sb,Fe,Ge)S.sub.4 PA0 Enargite series, Cu.sub.3 (As,Sb)S.sub.4 PA0 Fahlerz series, (Cu,Ag).sub.12 (Cu,Ag,Fe,Ge,Hg,Sn).sub.12 (As,Sb,Bi).sub.8 S.sub.26 PA0 Cubanite series, (Cu,Ag)Fe.sub.2 S.sub.3 PA0 Red glance series, Ag.sub.3 (As,Sb)S.sub.3 PA0 Stephanite group, Ag.sub.5 SbS.sub.4, Ag.sub.3 Bi(S,Te).sub.3, AgBi.sub.3 S.sub.5 PA0 Andonite group, Pb.sub.2 Ag.sub.2 Sb.sub.6 S.sub.12
Certain essential process stages are discussed below. By using the separation process based on the specific gravity difference between noble metals and the gangue, it is possible to obtain in the concentrate those coarse fractions of the gold minerals and gold which, being large in size and small in surface area, retard the cyanide leach. The recovery of gold by these processes is high. Yield values of 11-19% and 28-73% are mentioned for African and Australian refining plants, respectively.
Apparatus for concentration based on the specific gravity principle are numerous; some examples: Corduroy tables and gutters, grooved-belt concentrators, vibrating tables, Jig concentrators, Johnson's cylinder, etc.
The concentrate obtained from the separation stage 2 is amalgamated. Before the adoption of the method of using cyanide, all gold was separated by amalgamation. The amalgamation plant then comprised a stamp mill, as well as amalgamated silver-surfaced copper sheets used for amalgamation. Later, amalgam sheets were also used in the Corduroy gutter and similar apparatus. Nowadays, drum systems are used which allow the use of amalgamation activators. The amalgamation process is inhibited by dissolved sulfides, frothing agents, oils, fats, gold-covering layers, etc.
About 28-73% of the total gold content of the ore is recovered by means of amalgamation (on the average, 43% in African plants)
The residue obtained from the separation stage 2 is cyanided as such, if elements or compounds harmful to leaching are not present (quartz ores: Witwatersrand System). When gold is present in the ore in a finely-divided form, it can be cyanided without using pre-treatment methods (Carlin, Nev., U.S.A.). As well known, native gold and silver, their alloys and certain compounds dissolve when mixed in the presence of oxygen in alkalic cyanide solutions. The dissolving reaction as regards gold is EQU 2Au+4CN.sup.- +O.sub.2 +2H.sub.2 O.revreaction.2Au(CN).sub.2.sup.- +H.sub.2 O.sub.2 +2OH.sup.-
With a cyanide concentration of 0.02-0.08% by weight NaCN, the time required for the leaching is 6-72 hours (Kalgoorlie: 0.06-0.15% by weight NaCN, 6-88 hours). Tellurides, silver and silver compounds dissolve slowly. The rate of dissolving of gold is strongly dependent on the degree of grinding, particle size and covering on its surface, which may increase the above-mentioned leaching periods to manyfold.
If the residue from the separation stage 2 contains a large amount of sulfur compounds, selenides, tellurides, arsenic and sulfo-salts containing antimony and bismuth, etc. [Barberton Mountain Land, Kalgoorlie], this residue is froth-floated in order to remove the gangue minerals low in valuable metals.
The concentrate obtained, which contains the sulfides and other compounds, is roasted. The roasting must be carried out very carefully and under controlled conditions. The sulfur of the concentrate must be oxidized quantitatively and in such a manner that a soluble sulfate is obtained from the copper, that alkalic ferric sulfate is not produced (cyanicide), and that iron oxidizes to hematite. Hematite produced at a low temperature is porous, and sub-microscopic or otherwise enclosed gold is thus leachable. Impervious magnetite must not form, and therefore the oxygen pressure in the system must be controlled. Above 600.degree. C., hematite also begins to become more impervious.
The following values have been obtained as losses of gold as a fraction of the temperature when roasting thioarsenide (26.85 Fe, 15.52 As, 19.30 S, 0.20 Cu, 0.16 Sb) (loss, %/temperature, .degree.C.): 18.8/615, 28.1/700 and 33.7/802. [J. V. N. Dorr, S. L. Boosqui: Cyanidation and Concentration of Gold and Silver Ores, New York 1950, 170].
During roasting, the covering layer formed on the noble-metal surfaces by the collector agent is removed, but soluble sulfur, iron, arsenates, bismuth (covering layer risk), thiosulfates, etc., are often left in the product. The product of roasting must be washed very carefully before cyanidation.